The Park Tool CC-4.2 Chain Checker is a simple gauge for measuring chain wear, helping riders know exactly when to replace a chain. A worn, stretched chain accelerates wear on cassettes and chainrings, so replacing the chain on time saves money on more expensive drivetrain parts. The tool drops onto the chain and indicates wear levels at a glance, with no measuring or math required. It works with most derailleur chains. It is an inexpensive tool that pays for itself by extending drivetrain life. It suits every home workshop.